Ab initio study of magnetic properties in the adsorption of transition-metal atoms on arsenene

The magnetic properties of adsorption of different transition-metal (TM) atoms (Co, Cu, Mn, Fe, and Ni) on arsenene are investigated using density functional theory (DFT). Magnetism appears in the cases of Co, Mn, and Fe. Among all the magnetic cases, the TM atom prefers the same adsorption site. Then, we further study the interaction in two-TM-adsorbed system and different magnetic states are observed. Our results show that both nonmagnetic and ferromagnetic states exist in two-Co-adsorbed system and the p-d hybridization mechanism results in its ferromagnetic state. However, for two Mn and two Fe adsorbed systems, an AFM interaction is found, which could be reasonably explained by the superexchange mechanism. Such multiple magnetic properties may suggest promising applications of TM-adsorbed arsenene in the future.     

Magnetization of beryllium oxide in the presence of nonmagnetic impurities: Boron,carbon, and nitrogen

The electronic and magnetic states of a nonmagnetic insulator, namely, beryllium oxide, doped with nonmagnetic 2p elements (boron, carbon, and nitrogen) are studied using the density functional theory. The spin polarization of the 2p impurity states, as well as the transition of the doped BeO:(B,C,N) systems to the states of semiconducting or half-metallic magnets, is observed. The prospects for creating new magnetic materials by doping nonmagnetic insulators with nonmagnetic p impurities are discussed.     

First principles study on magnetic properties in ZnS doped with palladium

The electronic structures and magnetic properties in zinc-blende structure ZnS doped with nonmagnetic noble metal palladium have been investigated by means of density functional theory (DFT) calculations employing the generalised gradient approximation (GGA) and the GGA plus Hubbard U (GGA + U). Both the GGA and GGA + U methods demonstrate half-metallicity in Pd-doped ZnS with total magnetic moments of about 2.0μ _B per supercell. The half-metallic ferromagnetism stems from the hybridisation between Pd-4d and S-3p states and could be attributed to a double-exchange mechanism. These results suggest a recipe for obtaining a promising dilute magnetic semiconductor by doping nonmagnetic 4d elements in ZnS matrix.     

Exchange Bias in Layered GdBaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript> Cobaltite

It is established that excess oxygen content δ influences the exchange bias (EB) in layered GdBa-Co₂O_{5 + δ} cobaltite. The EB effect arises in p-type (δ > 0.5) cobaltite and disappears in n-type (δ < 0.5) cobaltite. The main parameters of EB in GdBaCo₂O_5.52(2) polycrystals are determined, including the field and temperature dependences of EB field H _EB , blocking temperature T _B , exchange coupling energy J _i of antiferromagnet–ferromagnet (AFM–FM) interface, and dimensions of FM clusters. The training effect inherent in systems with EB has been studied. The results are explained in terms of exchange interaction between the FM and AFM phases. It is assumed that the EB originates from the coexistence of Co³⁺ and Co⁴⁺ ions that leads to the formation of monodomain FM clusters in the AFM matrix of cobaltite.     

Electronic transport through a Majorana bound state coupled to a T-shaped quantum-dot system with Coulomb interaction

Using the Green’s function technique, we respectively investigate the electron transport properties of two spin components through the system of a T-shaped double quantum dot structure coupled to a Majorana bound state, in which only one quantum dot is connected with two metallic leads. We explore the interplay between the Fano effect and the MBSs for different dot-MBS coupling strength λ, dot-dot coupling strength t, and MBS-MBS coupling strength ε_M in the noninteracting case. Then the Coulomb interaction and magnetic field effect on the conductance spectra are investigated. Our results indicate that G_↓(ω) is not affected by the Majorana bound states, but a “0.5” conductance signature occurs in the vicinities of Fermi level of G_↑(ω). This robust property persists for a wide range of dot-dot coupling strength and dot-MBS coupling strength, but it can be destroyed by Coulomb interaction in quantum dots. By adjusting the size and direction of magnetic field around the quantum dots, the “0.5” conductance signature damaged by U can be restored. At last, the spin magnetic moments of two dots by applying external magnetic field are also predicted.     

A semiempirical calculation of the fine and Zeeman structure of 4<Emphasis Type="Italic">p</Emphasis>4<Emphasis Type="Italic">f</Emphasis> and 4<Emphasis Type="Italic">p</Emphasis>5<Emphasis Type="Italic">f</Emphasis> configurations of Ge I. Gyromagnetic ratios

The gyromagnetic ratios (g-factors) belong to the most important characteristics of atoms. For the 4p4f configuration of a germanium atom experimental values of g-factors are available only for four levels, while similar experimental data on the 4p5f configuration of Ge I are absent. Therefore, a theoretical study of the fine and Zeeman structures is topical for determining the gyromagnetic ratios. All the calculations are performed in the one-configuration approximation with the energy-operator matrix containing a maximum possible number of interactions, including magnetic: spin-orbit (own and other), spin-spin, and also orbitorbit interaction. The fine structure has been examined in three (LS, LK, and jK) approximations in order to establish the nature of coupling in the systems studied and the reliability of g-factors. Apart from the g-factors, in studying the Zeeman splitting, its specific features—the crossing and anticrossing fields of magnetic components— have been determined. A comparative analysis of g-factors was performed that showed that our results are in agreement with the available, albeit few in number, experimental data. At all stages, the corresponding energy-operator matrices were numerically diagonalized, i.e., all the results presented in the paper were obtained in the intermediate coupling scheme.     

Combined analysis of static and dynamic magnetic characteristics of multilayer CoFeZr/α-Si nanostructures

The combined method of static and dynamic magnetic measurements is used to study nano-dimensional multilayer amorphous CoFeZr/α -Si films. It is established that at the thickness of magnetic layers x = 5–12 nm their magnetization does not differ from that of the bulk material. It is shown that as x is lowered to 2–3 nm, the magnetization of magnetic layers decreases, which may be due to the formation of mixed layers containing nonmagnetic silicides. At a thickness of nonmagnetic interlayers of less than 1 nm the features characteristic of a weak antiferromagnetic interaction of neighboring layers are observed.     

Tunable magnetic interaction by an applied electric field in a Co-adsorbed SiC monolayer

The magnetic properties of Co-adsorbed SiC monolayer under an external electric field are investigated using first-principles method. In the absence of the electric field, the interaction between two Co atoms is ferromagnetic, which is originated by the p?d hybridization between Co and its neighboring C and Si atoms. When an electric field was introduced along the c axis, the interaction between two Co dopants switched from ferromagnetic to antiferromagnetic, which could be dominated by the competition between p?d exchange and superexchange. Moreover, the magnetic anisotropy prefers to parallel to the a axis and it seems not to be turn into the c axis under the electric field.     

Peculiarities of the magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Sm<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

Manganites of the Sm1?xSr_xMnO₃ system (x=0.33, 0.4, and 0.45) possess giant negative values of the magnetoresistance Δρ/ρ and the volume magnetostriction ω near the Curie temperature T_C. In the compound with x=0.33, the isotherms of Δρ/ρ, ω, and magnetization σ exhibit smooth variation and do not reach saturation up to maximum magnetic field strengths (120 kOe) studied (according to the neutron diffraction data, this substance comprises a ferromagnetic (FM) matrix with distributed clusters of a layered antiferromagnetic (AFM) structure of the A type). In the compounds with x=0.4 and 0.45 containing, besides the FM matrix and A-type AFM phase, a charge-ordered AFM phase of the CE type (thermally stable to higher temperatures as compared to the A-type AFM and the FM phases), the same isotherms measured at T ≥ T_C show a jumplike increase in the interval of field strengths between H_c1 and H_c2 and then reach saturation. In the interval H_c1 > H > H_c2, the σ, ω, and Δρ/ρ values exhibit a metastable behavior. At temperatures above T_C, the anisotropic magnetostriction changes sign, which is indicative of rearrangements in the crystal structure. The giant values of ω and Δρ/ρ observed at T ≥ T_C for all compounds, together with excess (relative to the linear) thermal expansion and a maximum on the ρ(T) curve, are explained by the phenomenon of electron phase separation caused by a strong s-d exchange. The giant values of magnetoresistance and volume magnetostriction (with ω reaching ～10^?3) are attributed to an increase in the volume of the FM phase induced by the applied magnetic field. In the compound with x=0.33, this increase proceeds smoothly as the FM phase grows through the FM layers in the A-type AFM phase. In the compounds with x=0.4 and 0.45, the FM phase volume increases at the expense of the charge-ordered CE-type AFM structure (in which spins of the neighboring manganese ions possess an AFM order). The jumps observed on the σ(H) curves, whereby the magnetization σ reaches ～70% of the value at T=1.5 K, are indicative of a threshold character of the charge-ordered phase transition to the FM state. Thus, the giant values of ω and Δρ/ρ are inherent in the FM state, appearing as a result of the magnetic-field-induced transition of the charge-ordered phase to the FM state, rather than being caused by melting of this phase.     

High-pressure effects on the crystal and magnetic structure of managanites

Nuetron diffraction studies of high-pressure effects on the crystal and magnetic structure of A_1-x A _x ^′ MnO₃ manganites (A = Pr, La; A′ = Sr, Ca, Na) are reviewed. High pressure leads to various changes in the magnetic structure of manganites: the appearance of a new A-type antiferromagnetic (AFM) state in compounds with the initial ferromagnetic (FM) or pseudo-CE type AFM state, such as La_{1 ? x} Ca _x MnO₃ (x = 0.25, 0.33), Pr_{1 ? x} Sr _x MnO₃ (x = 0.48), Pr_0.7Ca_0.3Mn_{1 ? y} Fe _y O₃ (y = 0, 0.1), Pr_{1 ? x} Na _x MnO₃ (x = 0.2, 0.25); and the appearance of a new C-type AFM state in the Pr_0.44Sr_0.56MnO₃ compound with the initial A-type AFM state. The observed changes in the magnetic structure and the behavior of the transition temperature to the FM state under high pressure are discussed in the framework of the current theoretical concepts.     

Magnetic phase diagram and correlation between metamagnetism and superconductivity in Ru<Subscript>0.9</Subscript>Sr<Subscript>2</Subscript>YCu<Subscript>2.1</Subscript>O<Subscript>7.9</Subscript>

The magnetic superconductorRu_0.9Sr₂YCu_2.1O_7.9 (Ru-1212Y) has beeninvestigated using neutron diffraction under variable temperature and magnetic field. Withthe complementary information from magnetization measurements, we propose a magnetic phasediagram T-H for the Ru-1212 system. Uniaxialantiferromagnetic (AFM) order of 1.2μ _B /Ruatoms with moments parallel to the c-axis is found below the magnetictransition temperature at  ~140 K in the absence of magnetic field. In addition,ferromagnetism (FM) in the ab-plane develops below  ~120 K, butis suppressed at lower temperature by superconducting correlations. Externally appliedmagnetic fields cause Ru-moments to realign from the c-axis to theab-plane, i.e. along the ?1,1,0? direction, and induce ferromagnetismin the plane with  ~1μ _B at 60 kOe.These observations of the weak ferromagnetism suppressed by superconductivity and thefield-induced metamagnetic transition between AFM and FM demonstrate not only competingorders of superconductivity and magnetism, but also suggest a certain vortex dynamicscontributing to these magnetic transitions.     

High-pressure induced magnetoresistance in <Emphasis Type="Italic">p</Emphasis>-InAs:Mn and <Emphasis Type="Italic">p</Emphasis>-CdGeAs<Subscript>2</Subscript>:Mn

The baric (P ≤ 5GPa) and magnetic-field (H ≤ 5 kOe) dependences of the transverse magnetore-sistance Δρ _xx /ρ₀ have been measured for p-InAs (R _H = 22.5 cm³/C, ρ = 0.15 Ω cm) and the new ferromag-netic semiconductor p-CdGeAs₂ (R _H = 5 cm³/C, ρ = 0.62 Ω cm), doped with a magnetic impurity (Mn), near the temperature T = 297 K. The dependences Δρ _xx /ρ₀ (P, H) for p-InAs:Mn and p-CdGeAs₂:Mn exhibit a magnetoresistive effect with an increase in pressure, and a pressure-induced magnetoresistance hysteresis is observed in p-CdGeAs₂:Mn with a pressure drop.     

Thermoelectric figure of merit in a quantum wire coupled to a graphene sheet between ferromagnetic leads

We theoretically investigate the figure of merit ZT for a quantum wireside-coupled by a graphene sheet and sandwiched between two ferromagnetic electrodes withnoncollinear magnetic moments. By using the nonequilibrium Green’s function combining withthe tight-binding Hamiltonian, we demonstrate that the ZT for the system developsan oscillating behavior and weakly depends on the wire-graphene coupling strength as wellas magnetic configuration of the leads. On the contrary, it is strongly dependent ontemperature and the polarization strength of the leads. Importantly, the maximum value ofZT for thesystem without the polarization strength (p = 0) is about 1.1 at temperature k _B T =0.015Γ ₀, which is in agreement with theexperimental measurements for silicon nanowires.     

Phase transitions and magnetoelectric coupling in BiFe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> multiferroics

Multiferroic BiFe_1?xZn _x O₃ ceramics were prepared by solution combustion method. Their structure, magnetoelectric, dielectric, magnetic, thermal characteristics were studied. The magnetic M(T) and heat capacity C _p (T) measurements demonstrate an antiferromagnetic to paramagnetic phase transition (T _N ) around 635 K. The anomaly on the temperature dependence of the dielectric constant near T _N was observed, which could be induced by the magnetoelectric coupling between electric and magnetic ordering. The magnetoelectric behavior was also confirmed by the linear relation between Δε and M², which is in the agreement of the Ginzburg-Landau theory for the second-order phase transition.     

Full t-matrix approach to quasiparticle interference in non-centrosymmetric superconductors

We develop the full t-matrix theory of quasiparticle interference (QPI)for non-centrosymmetric (NCS) superconductors with Rashba spin-orbit coupling. We give aclosed solution for the QPI spectrum for arbitrary combination and strength of nonmagnetic(V _c ) and magnetic(V _m ) impurity scattering potentials interms of integrated normal and anomalous Green’s functions. The theory is applied to arealistic 2D model of the Ce-based 131-type heavy fermion superconductors. We discuss theQPI dependence on frequency, composition and strength of scattering and compare with Bornapproximation results. We show that the QPI pattern is remarkably stable against changesin the scattering model and can therefore give reliable information on the properties ofRashba-split Fermi surface sheets and in particular on the accidental nodal position ofthe mixed singlet-triplet gap function in NCS superconductors.     

Mössbauer study of copper ferrite diluted by Ga<Superscript>3+</Superscript> and Al<Superscript>3+</Superscript> ions

Mössbauer spectra of the CuGa _x Al_2x Fe_{2 ? 3x} O₄ system (x = 0.1, 0.2, 0.3, 0.4, 0.5) have been studied at a temperature of 295 K. The results obtained are compared with Mössbauer data for the CuGa _x Al _x Fe_2?2x O₄ system. It is established that the hyperfine magnetic fields H _B (for octahedral sites) and H _A (for tetrahedral sites) for ferrites in both systems with ferrimagnetic ordering decrease linearly depending on the total number of nonmagnetic ions in octahedral and tetrahedral sites of the two systems.     

Negatively charged excitons in semimagnetic CdSe/ZnSe/ZnMnSe quantum dots

Low-temperature (T = 1.6 K) photoluminescence (PL) of individual CdSe/ZnSe/ZnMnSe quantum dots (QDs) with different magnitudes of the sp-d exchange interaction between the magnetic impurity ions and charge carriers has been studied in a magnetic field up to 12 T applied in the Faraday and Voigt geometry. The magnitude of the interaction was controlled by changing the fraction (η_{e, h}) of the squared wave function of charge carriers in the semimagnetic barrier by means of variation of the nonmagnetic (ZnSe) layer thickness. It is established that the sp-d exchange interaction leads to a change in the sign of the effective hole g factor even for η_{e, h} ～ 5%, while further increase in the interaction magnitude is accompanied by a rapid growth in the magnitude of spin splitting for both electrons and holes. The quantum yield of PL exhibits a significant decrease due to nonradiative Auger recombination with the excitation of Mn ions only for η_{e, h} ～ 12%, while the rate of the holes spin relaxation starts growing only for still higher η_{e, h} values. In a strong magnetic field perpendicular to the sample plane, the alignment of Mn spins leads to suppression of the Auger recombination only in the excited spin state. For a small rate of the hole spin relaxation, this leads to a rather unusual result: the emission from an excited trion state predominates in strong magnetic fields.     

Microwave photoresistance of a two-dimensional electron gas in a ballistic microbar

The effect of millimeter microwave radiation on the electron transport of two-dimensional (2D) ballistic microbars formed on the basis of individual GaAs quantum wells at a temperature of T = 4.2 K in magnetic fields B < 0.6 T has been investigated. Differences have been revealed in the magnetic field dependences of the microwave photoresistance of a 2D electron gas in Hall bars with a length L and a width W for the cases L, W > l _p and L, W < l _p , where l _p is the electron mean free path for momentum. The microwave photoresistance in macroscopic bars (L, W > l _p ) is a periodic alternating function of the inverse magnetic field; in microbars (L, W < l _p ), it is a periodic positive function of 1/B. The experimental results indicate that the mechanisms of the microwave photoresistance of a 2D electron gas are different for macroscopic and microscopic bars.     

Specific heat of La(Fe<Subscript>0.873</Subscript>Co<Subscript>0.007</Subscript>Al<Subscript>0.12</Subscript>)<Subscript>13</Subscript> compound in antiferromagnetic and ferromagnetic states

The low-temperature specific heat C _p of La(Fe_0.873Co_0.007Al_0.12)₁₃ compound has been measured in two states: (i) antiferromagnetic (AFM) with a Néel temperature of T _N = 192 K and (ii) ferromagnetic (FM). The FM order appears at T = 4.2 K in a sample exposed to an external magnetic field with induction B _C ≥ 2.5 T and is retained for a long time in a zero field at temperatures up to T*_C = 23 K. The coefficient γ_FM in the low-temperature specific heat C = γT + βT ³ in the FM state differs quite insignificantly from that (γ_AFM) in the AFM state. Contributions to the low-temperature specific heat, which are related to a change in the elastic and magnetoelastic energy caused by magnetostrictive deformations, are considered.